In dentistry, practitioners use a variety of restorative materials to create crowns, veneers, direct fillings, inlays, onlays and splints. One of the major goals in restorative dentistry is to produce restorations that match the esthetics of the natural tooth. Highly esthetic tooth colored restorations were first introduced to dentistry in the 1940's with acrylic resins and silicate cements. These were direct filling restorations that were tooth colored and translucent in visible light like natural teeth. When placed in the mouth, the fillings were not easily discernible from the tooth itself. In the 1950's, dental porcelains were introduced, which provided a variety of shades and translucencies to further improve the esthetics of the restorations. These were used in restorations, such as porcelain fused to metal crowns and bridges, or in inlays, onlays and veneers. Tooth shading with porcelain restorations has been highly successful and has become state-of-art in the industry today. In the 1970's, fluorescence was incorporated into dental porcelains, which further improved the esthetics of dental restorations and made them more natural appearing, especially under fluorescent lighting conditions. More recently, in the 1990's, opalescence has been incorporated into dental porcelains to produce the natural "opal effect" present in natural teeth.
Translucency, shading, fluorescence and opalescence are optical properties that give the natural tooth its vital-looking appearance. Translucency and shading have the greatest impact on the total vitality of the tooth because they are the most readily observed. Dentin and enamel are both translucent, but enamel is more translucent, almost transparent and colorless. The color or shade of the tooth mostly comes from dentin and is transmitted through the enamel layer to the surface of the tooth. Enamel is a highly mineralized crystalline structure composed of millions of enamel rods or prisms. As light travels through the enamel, the rods scatter and transmit the rays to the tooth surface much like a fiber optic system. Enamel, though highly transparent, does not transmit light like a clear window glass. Instead, the enamel diffuses the light, rendering the enamel opalescent.
Fluorescence and opalescence are more subtle optical properties that further enhance the natural-looking, life-like appearance or "vitality" of the tooth. Fluorescence is defined as the emission of electromagnetic radiation that is caused by the flow of some form of energy into the emitting body, which ceases abruptly when the excitation ceases. In natural teeth, components of the enamel, including hydroxyapatite, fluoresce under long wavelength ultraviolet light, emitting a white visible light. This phenomenon is subtle in natural daylight but still adds further to the vitality of the tooth. In contrast, under certain lighting conditions, the lack of fluorescence in a restorative material may become alarming. Under "black light" conditions, such as that often used in discotheque-type night clubs, if a restoration does not fluoresce, the contrast between the tooth and restoration may be so great that the tooth may actually appear to be missing.
Opalescence is defined as the milky, iridescent appearance of a dense, transparent medium or colloidal system when illuminated by visible light. It is best illustrated by the mineral opal, which is a natural hydrated form of silica. The "opal effect" is a light scattering phenomenon in translucent materials that produces a blue effect in reflected light due to the scattering of short wavelength light and an orange effect in transmitted light. This effect is different from simple reflected light in translucent materials and produces the milky iridescent effect present in the natural tooth. Restorations that are not opalescent do not have the vital looking appearance of a natural tooth itself.
Without being bound by theory, the chemistry and structure of enamel may be responsible for the "opal effect." Chemically, tooth enamel is a highly mineralized crystalline structure containing from 90% to 92% hydroxy apatite by volume. Structurally, it is composed of millions of enamel rods or prisms aligned perpendicular to the dentinoenamel junction and extending to the tooth surface. The enamel rods measure about 4-8 .mu.m in diameter and the head or body section at the surface of the rods is about 5 .mu.m wide. The crystallites are tightly packed in a distinct pattern or orientation that gives strength, hardness and structural identity to the enamel prisms. The particle size and crystalline orientation of the enamel prisms likely are responsible for producing the light scattering "opal effect."
Although opalescence has been incorporated into dental porcelains, the current trend in dental restorative technology is to use composite resins for restoration, rather than the porcelains. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a resin matrix. These materials may be dispersion-reinforced, particulate-reinforced, or hybrid composites.
Dispersion-reinforced composites include a reinforcing filler of, for example, fumed silica having a mean particle size of about 0.05 .mu.m or less, with a filler loading of about 30%-45% by volume. Because of the small particle size and high surface area of the filler, the filler loading into the resin is limited by the ability of the resin to wet the filler. Consequently, the filler loading is limited to about 45% by volume. Due to the low loading, the filler particles are not substantially in contact with one another. Thus, the primary reinforcing mechanism of such dispersion-reinforced composites is by dislocation of flaws in the matrix around the filler. In dispersion-reinforced materials, the strength of the resin matrix contributes significantly to the total strength of the composite. In dentistry, dispersion-reinforced composite resins or microfills are typically used for cosmetic restorations due to their ability to retain surface luster. Typically, these microfill resins use free radical-polymerizable resins such as methacrylate monomers, which, after polymerization, are much weaker than the dispersed filler. Despite the dispersion reinforcement, microfill resins are structurally weak, limiting their use to low stress restorations.
One example of a dispersion-reinforced composite is HELIOMOLAR.RTM., which is a dental composite including fumed silica particles on the order of 0.05 .mu.m mean particle size and rare earth fluoride particle on the order of less than 0.2 .mu.m mean particle size. HELIOMOLAR.RTM. is a radiopaque microfill-type composite available from Vivadent. The rare earth fluoride particles contribute to both flexural strength and radiopacity.
Particulate-reinforced composites typically include a reinforcing filler having an average particle size greater than about 0.6 .mu.m and a filler loading of about 60% by volume. At these high filler loadings, the filler particles begin to contact one another and contribute substantially to the reinforcing mechanism due to the interaction of the particles with one another and to interruption of flaws by the particles themselves. These particulate-reinforced composite resins are stronger than microfill resins. As with the dispersion-reinforced composites, the resin matrix typically includes methacrylate monomers. However, the filler in particulate-reinforced composites has a greater impact on the total strength of the composite. Therefore, particulate-reinforced composites are typically used for stress bearing restorations.
Another class of dental composites, known as hybrid composites, include the features and advantages of dispersion reinforcement and those of particulate reinforcement. Hybrid composite resins contain fillers having an average particle size of 0.6 .mu.m or greater with a microfiller having an average particle size of about 0.05 .mu.m or less. HERCULITE.RTM. XRV (Kerr Corp.) is one such example. HERCULITE.RTM. is considered by many as an industry standard for hybrid composites. It has an average particle size of 0.84 .mu.m and a filler loading of 57.5% by volume. The filler is produced by a wet milling process that produces fine particles that are substantially contaminant free. About 10% by volume of this filler exceeds 1.50 .mu.m in average particle size. In clinical use, the surface of HERCULITE.RTM. turns to a semi-glossy matte finish over time. Because of this, the restoration may become distinguishable from normal tooth structure when dry, which is not desirable for a cosmetic restoration.
Another class of composites, flowable composites, have a volume fraction of structural filler of about 10% to about 30% by volume. These flowable composites are mainly used in low viscosity applications to obtain good adaptation and to prevent the formation of gaps during the filling of a cavity.
Various methods of forming submicron particles, such as precipitation or sol gel methods, are available to produce particulate reinforcing fillers for hybrid composites. Comminution by a milling method may also be used for forming the submicron particles. The predominant types of milling methods are dry milling and wet milling. In dry milling, air or an inert gas is used to keep particles in suspension. However, fine particles tend to agglomerate in response to van der Waals forces, which limits the capabilities of dry milling. Wet milling uses a liquid such as water or alcohol to control agglomeration of fine particles. Therefore, wet milling is typically used for comminution of submicron-sized particles. As opposed to the spherically-shaped particles typically produced by sol gel methods, the ground particles are nonspherical, providing increased adhesion of the resin to the structural filler, thereby further enhancing the overall strength of the composite.
In co-pending U.S. patent application Ser. No. 09/270,999 now U.S. Pat. No. 6,121,344, entitled "Optimum Particle Sized Hybrid Composite," C. Angeletakis et al., filed on Mar. 17, 1999 and incorporated herein by reference in its entirety, there is disclosed a resin-containing dental composite including a translucent structural filler of ground particles having an average particle size of between about 0.05 .mu.m and about 0.5 .mu.m that has the high strength required for load bearing restorations, yet maintains a glossy appearance in clinical use required for cosmetic restorations. Specifically, since the structural filler size is less than the wavelength of visible light, the surface of a dental restoration will reflect more light in some directions than in others even after wear of the composite by brushing. The visible light waves do not substantially interact with the structural filler particles protruding out of the surface of the composite, and therefore, haze is reduced and the luster of the surface is maintained even after substantial brushing. This application represents a significant advancement in hybrid composite technology, but some of the composites produced according to the teachings of this pending application lack the vital looking appearance of a natural tooth.
As can be discerned from the multitude of patents in the area of dental restorative materials, the development of composite resins for dental restorations has been extremely difficult, attempting to balance physical properties with optical properties to produce an overall superior product. Pursuit of the "opal effect" in composite resins has mainly focused on small additions of "opal agents" or pigments, such as microfine titania, alumina or zirconia to achieve opalescence. For example, European Publication No. 533,434 describes the addition of microfine titania (&lt;0.2.mu.m) in an amount less than 2 wt. % to hybrid or microfill cold-polymerizable dental composite formulations to achieve opalescence.
It is desirable to achieve opalescence in both hot- and cold-polymerizable dental composite resins, particularly the composite described in the copending application Ser. No. 09/270,999. While opalescence may be achieved by modifying the composite formulations with small amounts of opal agents or pigments, the present invention focuses on the development of a self-opalescing composite resin.